Polypropylene and polyethylene are relatively inexpensive, but has a lower thermal resistance. Therefore, these materials have limitations in applications requiring excellent thermal resistance such as electronic components and electrical equipment. In order to increase the mechanical strength and thermal resistance of thermoplastic resins including polyolefins, such as polypropylene, and polyvinyl chloride, attempts have been made to increase the crystallinity thereof. For example, there is a known method in which a melt of a thermoplastic resin is prepared and then slowly cooled. However, such a method presents a problem in that the degree of crystallization cannot sufficiently be increased.
Patent Literature 1 below discloses a method for producing a polymer-oriented crystalline body which can further increase the degree of crystallization. In Patent Literature 1, a polymer melt, such as a polypropylene melt, is stretched at a strain rate more than the critical stretching strain rate to put it into an oriented melt state. Then, the polymer melt is cooled and crystallized while being maintained in an oriented melt state. The literature states that a sheet-like polymer-oriented crystalline body increased in degree of crystallization can be thus obtained.